[dpdk-dev] [RFC][PATCH 2/5] pci: allow shared device instances.

[Declan Doherty]

Hey Thomas, I've been working on this with Radu, so see my take below

On 10/05/2017 11:28 AM, Thomas Monjalon wrote:
> 10/05/2017 12:11, Radu Nicolau:
>> Hi
>>
>>
>> On 5/10/2017 10:09 AM, Thomas Monjalon wrote:
>>> Hi,
>>>
>>> 09/05/2017 16:57, Radu Nicolau:
>>>> Updated PCI initialization code to allow devices to be shared across multiple PMDs.
>>>>
>>>> Signed-off-by: Radu Nicolau <radu.nicolau at intel.com>
>>> I was waiting the day when we have a device shared
>>> by two different interfaces.
>>> Note that some Mellanox and Chelsio devices already instantiate
>>> two ethdev ports per PCI device.
>>>
>>> Please explain your idea behind this "shared" flag.
>>> What is your exact need?
>>
>> Currently for each pci device a look-up into a list of PMDs is
>> performed, and when a match is found the system moves to the next
>> device. Having this flag will allow a PMD to inform the system that
>> there may be more matches, more PMDs that can be used for this
>> particular device.
>> There is a difference when comparing to the devices you mentioned above,
>> in this case the PMDs are totally different types, one network and one
>> cryptodev PMD for each IXGBE network card.
>
> Yes I know it is a lack in DPDK.
> Linux introduced MultiFunction Device in 2005:
> 	http://events.linuxfoundation.org/sites/events/files/slides/belloni-mfd-regmap-syscon_0.pdf
>

So at the most basic level the intention is to allow more than one 
device of different types, in our case a net PMD and a crypto PMD, to be 
instantiated on a single PCI bar, in essence to share the bar. I'm not 
familiar with the approaches taken in the Mellanox and Chelsio devices 
but I assume they are handled with the driver probe/create  functions 
independently from the EAL infrastructure?

For the initial proto-typing of this RFC we only implemented the 
multi-device creation but I envisage that there will be a requirement 
for sharing state between drivers, or at a minimum implementing locking 
around shared resources, registers etc. And I would like to see this 
done in a generic fashion that can me leverage by any driver and not 
have each driver having to solve this independently.

>>> Do you think it is the best solution?
>>
>> We evaluated different approaches and this is what we settled on. It
>> might not be the best, if there are any suggestions of other ways to
>> achieve this I would be thankful.

I think this approach was sufficient to enable the RFC and kick off the 
discussion, but it is not a fully featured solution and we wanted to get 
community feedback before progressing to far along with a fully featured 
solution.


>
> Please could you explain the other approaches you thought
> with pros and cons?
>
> Thanks
>